This application relates to acoustic signals and acoustic ranging. More generally, the application is directed to acoustic ranging techniques suitable for use in seismic data acquisition, including, but not limited to, hydroacoustic ranging for marine-based seismic surveys.
Seismic surveys are used to investigate structures beneath the earth's surface, with broad applications in geology and geophysics, oceanography, and even archaeology. Seismic surveys are also important tools for both land-based and marine exploration techniques in the oil and gas industries, utilizing a range of different terrestrial arrays, ocean-bottom sensor nodes, and towed seismic streamers. In marine applications, the streamers each have a number of individual hydrophones or other acoustic receivers, distributed or spaced along the cable length.
Generally, seismic surveys are accomplished by firing air guns or other sources of acoustic or seismic energy, recording the responses of the receivers to subsurface reflections, and processing the data to obtain an image of the corresponding geological formations and other relevant physical structure. In marine surveys, multiple towed streamer cables are typically employed to obtain more detailed three-dimensional seismic data, over shorter periods of time.
Better data can yield more precise and detailed reproductions of the subsurface structure, but accurate receiver positioning is a critical element of the imaging process. Both absolute and relative position information are relevant, including not only source and receiver locations with respect to the subsurface structures of interest, but also distances between the sources and receivers themselves, and among the individual streamer cables and other elements of the array.
Typically, multiple streamer cables are towed behind a vessel in a more or less parallel configuration, with transmitter-to-receiver positioning obtained via acoustic ranging. Individual acoustic pulse transit times are determined from the receiver telemetry provided to a processor or controller (e.g., a navigational controller on the tow vessel), in order to obtain a position solution by converting the transit times into spatial separations.
Typically, a combination of acoustic ranging and radiopositioning is used to complete the determination of source-receiver distances, and to obtain relative positioning with respect to other elements in the array (e.g., distances to the tow vessel or vessels, floats, buoys, and other navigational and geodetic references). Depth sensors, electronic compasses, GPS systems and laser positioning systems are also used, in order to dynamically model the streamer geometry over a range of different towing and environmental conditions.
In large-scale marine surveys, streamer length is a major factor. Towed arrays may be distributed over several square kilometers of surface area, or more. Even where the tow vessel follows a more or less constant heading through the survey field, therefore, environmental factors like wind, waves and currents can have a substantial impact the streamer shape. Variations in tow velocity can also be an issue, as the arrays must sometimes be navigated around coastlines, other vessels, and other navigational hazards.
A complete positioning solution is desired for each shot point in the survey data, including the associated time intervals over which ranging signals are collected. Ideally, the positioning solution should be computed substantially simultaneously with the data collection, before the streamer shape and position can change. For larger arrays with acoustic transmitters and receivers distributed along tens of kilometers of total streamer cable, however, the number of potential acoustic range pairs is enormous, and a complete, idealized and simultaneous solution may be impractical. As a result, there remains a substantial need for more complete acoustic and hydroacoustic ranging techniques, which can provide improved positioning solutions over a wide range of seismic array distance scales, over as short a time window as practical.